Biology · Introductory biology · Worked example
Predict agglutination in blood typing
A drop of type B blood is mixed with anti-A reagent and another with anti-B reagent. Which drops clump? Then the plasma of a type O person is mixed with type A, type B and type O red cells. Which of those clump, and why?
Name the antigens on the cells
Type B red cells carry the B antigen and no A antigen.
Match each reagent to its epitope
Anti-B fits the B antigen, so it binds the type B cells. Its several binding sites bridge neighboring cells, and the drop clumps. Anti-A finds no A antigen, binds nothing, and that drop stays smooth.
Name the antibodies in type O plasma
Why does a type O person have antibodies at all? People make antibodies, mostly IgM, against the A and B antigens their own cells lack. Type O cells carry neither, so type O plasma holds both anti-A and anti-B.
Mix the plasma with each kind of cell
Anti-A clumps type A cells and anti-B clumps type B cells. Type O cells carry neither antigen, so nothing in the plasma binds them, and they stay smooth.
Read the result for transfusion
Clumping is what a mismatched transfusion would do inside a patient. Type O red cells are not clumped by A, B or O plasma, because they carry neither A nor B antigen.
Result
The type B blood clumps with anti-B and not with anti-A. Type O plasma, holding both anti-A and anti-B, clumps type A and type B red cells but not type O cells, which carry neither antigen.
Your turn
1. One like the example
A drop of type AB blood is mixed with anti-A reagent and another with anti-B reagent. Which drops clump, and which antibodies does this person’s plasma hold?
Show the answer and explanation
Both drops clump. The plasma holds neither anti-A nor anti-B.
Type AB cells carry both antigens, so each reagent finds its epitope and bridges the cells. A person makes antibodies only against the antigens their own cells lack, and an AB person lacks neither.
2. A variation
A type A patient’s plasma is mixed with type A, type B, type AB and type O red cells. Which ones clump?
Show the answer and explanation
Type B and type AB cells clump. Type A and type O cells do not.
A type A person makes anti-B. It binds every cell that carries the B antigen, which is type B and type AB. Type A and type O cells carry no B antigen, so nothing binds them.
3. Somewhere new
A lab replaces the anti-A reagent with Fab fragments made from anti-A IgG. What happens when the fragments are mixed with type A blood, and why?
Show the answer and explanation
The fragments bind the A antigen, but the drop does not clump.
Each Fab fragment keeps one binding site, so it can coat a red cell’s A antigens, but no fragment can hold two cells at once. Clumping needs a molecule with at least two sites, which is why the reagent uses whole antibodies.
Keep exploring
The antibody opens in Biology in agglutination mode, one antibody bridging two type A red cells by their A antigens. Remove one binding link and the tool warns that agglutination needs multiple bound antigens.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
- Schroeder and Cavacini (2010), Structure and function of immunoglobulins, Journal of Allergy and Clinical Immunology 125: S41
- Gu et al. (2025), Immunoglobulin: unraveling its complex web in aging, Frontiers in Immunology 16: 1690018
- Brandtzaeg (2013), Secretory IgA: designed for anti-microbial defense, Frontiers in Immunology 4: 222
- Jajosky et al. (2023), ABO blood group antigens and differential glycan expression, iScience 26: 105798
- OpenStax Biology 2e — Antibodies
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